2 Answers
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6.3.1. The ABBO is defined as the Best Bid and Offer prices as represented by an aggregation of relevant European Order Books as
defined by Turquoise. An external market’s Best Bid and Offer will
only be part of the ABBO when that market is in Continuous Trading.
Turquoise will advise for each security which external markets will
be included in ABBO by a Market Notice and on the Turquoise Website. Turquoise will provide a reasonable period of notice prior to
implementing any change.

6.3.2. If there are no orders in the relevant European Order Books as defined by Turquoise or if the markets are closed the ABBO is deemed
to be undefined.

6.3.3. The ABBO can be set in smaller tick sizes than Turquoise tick size.

In other words, it's the best price from all other exchanges. Ie, NBBO minus this exchange.